The designers of electronic systems have for many years thought of ways to design and improve semiconductor devices. The vacuum tube, once the mainstay of electronics, had limitations such as the mechanically fabricated structures inside the glass envelope preventing miniaturization and integration, and the thermionic cathode keeping the power drain high. There have recently been significant developments in this area that offer the opportunity of escaping the previous restraints. Semiconductor fabrication techniques can now be used to develop structures in microminiature form and integrate many of them together. Combining these microminiature structures with a field emission electron source one can now produce microminiature vacuum tube structures which do not require heated cathodes These structures being on the order of micrometers in size, permit the integration of many devices on a single substrate, just as many semiconductor devices are produced on a single chip.
The Vacuum Microelectronic Devices presently in use require several unique three-dimensional structures, which include, a vacuum space, a sharp, preferably less than 100 nm radius field emission tip, and the accurate alignment of tip inside an extraction/control electrode structure. Vacuum Microelectronic Devices include a field-emission cathode and add additional structures, such as, an extension of the vacuum space, an anode opposite the cathode tip, and there may or may not be additional accurately aligned control electrodes placed between the tip and the anode.
The field emission display elements that utilize these Vacuum Microelectronic Devices use the basic field emission structure and add additional structures, such as, an extension of the vacuum space, a phosphor surface opposite the cathode tip, and additional electrodes to collect and/or control the electron current. Groups of individual Vacuum Microelectronic Devices and/or display elements can be electrically interconnected during fabrication to form integrated circuits and/or displays.
Vacuum Microelectronic Devices have several unique features. They are expected to have sub pico second switching speeds and are thought by some to be the fastest electronic devices possible They will operate at temperatures ranging from near absolute zero to hundreds of degrees Celsius limited principally by their materials of construction. These structures can be made of almost any conductor and insulator material. They are intrinsically radiation hard. They are also very efficient because control is by charge and not by current flow, and the use of high field emitters eliminates the thermionic emission heaters of traditional vacuum devices.
In U.S. Pat. No. 4,721,885, and also in an article published by Ivor Brodie, "Physical Considerations in Vacuum Microelectronics Devices", IEEE Transactions on Electron Devices, Vol. 36, No. 11, pages 2641-2644 (November 1989), a field-emission microtriode is described. The triode consists of a metal cone attached to a metal or high-conductivity semiconductor base electrode. The height of the cone is given as "h", the radius of curvature at the cathode tip is "r". A metal anode is held at a distance "d" from the tip of the cone by a second insulating layer. The cone tip is at the center of a circular hole having a radius "a", in a gate (or first anode) electrode of thickness "t". When the appropriate positive potential difference is applied between the base electrode and the gate electrode, an electric field is generated at the cathode tip that allows electrons to tunnel through the tip into the vacuum space and move towards the anode. The field at the tip and, hence, the quantity of electrons emitted can be controlled by varying the gate potential.
While these Vacuum Microelectronic Devices can be made in almost any size and may have applications as discrete devices, their best performance and major application is expected to come from extreme miniaturization, large arrays, and complex very large scale integration of circuits.
Non-thermionic field emitters, field emission devices, and field emission displays are all known in the art. Since the fabrication of the field emission cathode structure is a critical element common to the devices mentioned, its art will be addressed first. The material (insulators and conductors/field emitters) are all deposited and processed by relatively common deposition and lithographic processing techniques with the single exception of a special sharp edge (blade) or point (tip) structure which is common to all field-emission cathodes. The art can be broadly classified into five categories, and these categories are primarily categorized by the methods used to form this sharp blade or tip.
The first category is one of the earliest categories in which the cathode tip structure is formed by the direct deposition of the material. An example of this type is exemplified in a paper by C. A. Spindt, "A Thin-Film Field-Emission Cathode", J. Appl. Phys., Vol. 39, No. 7, pages 3504-3505 (1968), in which sharp molybdenum cone-shaped emitters are formed inside holes in a molybdenum anode layer and on a molybdenum cathode layer. The two layers are separated by an insulating layer which has been etched away in the areas of the holes in the anode layer down to the cathode layer. The cones are formed by simultaneous normal and steep angle depositions of the molybdenum and alumina, respectively, onto the rotating substrate containing the anode and cathode layers. The newly deposited alumina is selectively removed. Similar work has also been disclosed in U.S. Pat. No. 3,755,704.
A second category is the use of orientation-dependent etching of single crystal materials such as silicon. The principle of the orientation-dependent etching is to preferentially attack a particular crystallographic face of a material. By using single crystal materials patterned with a masking material, the anisotropically etched areas will be bounded by the slow etching faces which intersect at well defined edges and points of the material's basic crystallographic shape. A suitable combination of etch, material, and orientation can result in very sharply defined points that can be used as field emitters. U.S. Pat. No. 3,665,241 issued to Spindt, et al., is an example of this method in which an etch mask of one or more islands is placed over a single-crystal material which is then etched using an etchant which attacks some of the crystallographic planes of the material faster than the others creating etch profiles bounded by the slow etching planes (an orientation-dependent etch). As the slow etching planes converge under the center of the mask, multifaceted geometric forms with sharp edges and points are formed whose shape is determined by the etchant, orientation of the crystal, and shape of the mask. Orientation-dependent anisotropic etching while an established method to create the tips can also have an adverse effect by making these sharp tips blunt (or reducing the radius of the cathode tip), thus reducing their effectiveness as field emitters, as discussed by Cade, N. A. et al., "Wet Etching of Cusp Structures for Field-Emission Devices," IEEE Transactions on Electron Devices, Vol. 36, No. 11, pages 2709-2714 (November 1989).
A third category uses isotropic etches to form the structure. Isotropic etches etch uniformly in all directions. When masked, the mask edge becomes the center point of an arc which outlines the classic isotropic etch profile under the masking material. The radius of the arc is equal to the etch depth. Etching around an isolated masked island allows the etch profile to converge on the center of the mask leaving a sharp tip of the unetched material which can be used as a field emitter. An example of this is exemplified in U.S. Pat. No. 3,998,678, issued to Shigeo Fukase, et al. An emitter material is masked using islands of a lithographically formed and etch resistant material. The emitter material is etched with an isotropic etchant which forms an isotropic etch profile (circular vertical profile with a radius extending under the resist from the edge). When the etch profile converges under the center of the mask from all sides, a sharp point or tip results.
A fourth category uses oxidation processes to form the Vacuum Microelectronic Device. Oxidation processes form a tip by oxidizing the emitter material Oxidation profiles under oxidation masks are virtually identical to isotropic etch profiles under masks and form the same tip structure as the profiles converge under a circular mask. When the oxidized material is removed the unoxidized tip can function as a field emitter. U.S. Pat. No. 3,970,887 issued to Smith et al. exemplifies this process. A substrate of electron emission material such as silicon is used. A thermally grown oxide layer is grown on the substrate and is then lithographically featured and etched to result in one or more islands of silicon dioxide. The substrate is then reoxidized during which the islands of previously formed oxide act to significantly retard the oxidation of the silicon under them. The resulting oxidation profile is very similar to the isotropic etch profile and similarly converges under the islands leaving a sharp point profile in the silicon which can be exposed by removing the oxide. Other masking material such as silicon nitride can be used to similarly retard the oxidation and produce the desired sharp tip profile.
A fifth category etches a pit which is the inverse of the desired sharply pointed shape in an expendable material which is used as a mold for the emitter material and then removed by etching. U.S. Pat. No. 4,307,507 issued to Gray et al exemplifies a limited embodiment of this technique. Holes in a masking material are lithographically formed on a single crystal silicon substrate. The substrate is orientation-dependent etched through the mask holes forming etch pits with the inverse of the desired pointed shape. The mask is removed and a layer of emission material is deposited over the surface filling the pits. The silicon of the mold is then etched away freeing the pointed replicas of the pits whose sharp points can be used as field emitters.
All of the emitter formation techniques mentioned above have several limitations. Orientation-dependent etching requires the use of a substrate of single crystal emitter material. Most all of them require the substrate to be made of or coated with the emitter material. Most all of them form the emitter first which complicates the fabrication of the subsequent electrode layers and the vacuum space needed for a fully functional Vacuum Microelectronic Device.
Sometimes the method used or the particular processing regime does not produce field emission tips of sufficiently small radius. The art includes some methods by which the tip can be sharpened to further reduce this radius. In a paper by Campisi et al, "Microfabrication Of Field Emission Devices For Vacuum Integrated Circuits Using Orientation Dependent Etching", Mat. Res. Soc. Symp. Proc., Vol. 76, pages 67-72 (1987), reports the sharpening of silicon tips by slowly etching them in an isotropic etch. Another paper entitled "A Progress Report On The Livermore Miniature Vacuum Tube Project", by W. J. Orvis et al, IEDM 89, pages 529-531 (1989), reports the sharpening of silicon tips by thermally oxidizing them and then etching away the oxide. U.S. Pat. No. 3,921,022, also discloses a novel method of providing multiple tips or tiplets at the tip of a conical or pyramidical shaped field emitter.
Various processes creating two or three electrode VMD structures been reported in the art. As an example a paper entitled "A Progress Report On The Livermore Miniature Vacuum Tube Project", by Orvis et al, IEDM, pages 529-531 (1989), describes a process in which silicon emitters formed by either orientation-dependent or isotropic etching are used. Lithographically featured doped polysilicon anode and grid layers are separated from the emitter and each other by layers of low density glass.
It is now possible as exemplified in Busta, H. H. et al. "Field Emission from Tungsten-Clad Silicon Pyramids", IEEE Transactions on Electron Devices, Vol. 36, No. 11, pages 2679-2685 (November 1989), to use coating or cladding on these cathode tips or pyramids to enhance or modify the cathode tip properties.
In this developing field of Vacuum Microelectronic Devices the art has also started to show how these field emission cathodes and extraction electrodes can be used in a practical application, such as, in a display applications. U.S. Pat. No. 4,857,799 issued to Spindt et al illustrates how a substrate containing field emitters and extraction electrodes can be joined to a separate transparent window which contains anode conductors and phosphor strips, all of which can work in concert to form a color display. Another color display device using vacuum microelectronic type structure was patented in U.S. Pat. No. 3,855,499.
This patent application also discloses an etch process which can significantly reduce the unwanted undercut for a Vacuum Microelectronic Device while still allowing the formation of bridge structures.
In summary a typical field emission Vacuum Microelectronic Devices are made up of a sharply pointed cathode, surrounded by a control and/or extraction electrode, and pointing toward an anode surface. The cathode tip could have a point or a blade profile. One of the key technologies in fabricating these devices is the formation of the sharp field emission (cathode) tip which has preferably a radius on the order of 10-100 nm. The most common methods of formation include orientation-dependent etching, isotropic etching, and thermal oxidation.